Hubble trains its eye on an astronomical mystery

Late last month, Comet Holmes (17P), inexplicably brightened over one million- …

At the end of October, Nobel Intent reported on a surprising astronomical event; periodic comet Holmes (17P) brightened over one million-fold over the course of a single day. In mid-October, Comet Holmes was a "nonentity" in between the orbits of Jupiter and Mars. Then, on the evening of October 24, backyard astronomers the world over (well, in the Northern Hemisphere) saw a new "star" in the constellation Perseus. At the time, no one was sure why this extreme brightening happened. Now, scientists have trained Hubble's Wide Field Planetary Camera 2 on Holmes to attempt to shed—or more accurately receive—light on this new turn of events.

Hubble first looked at Comet Holmes back in 1999. At the time, the nucleus of the comet was too small to measure directly. Astronomers were able to infer the nucleus' diameter through measurements of its brightness and found it to be about 3.4 kilometers—roughly the distance between the Arc de Triomphe and the glass pyramid that sits in the Louvre's courtyard, for our Parisian readers. Subsequent images of Holmes taken by Hubble on October 29th and 31st showed three "spurs" of dust, and then a dust outburst just west of the nucleus, respectively.

What the Hubble images do not show is a clear reason for the sudden brightening. In 2006, Hubble eyed comet Schwassmann-Wachmann 3 (SW3) eject a number of "mini-comets" after a sudden brightening. Ground based images of Holmes suggest that a large piece broke off and subsequently disintegrated into dust, which resulted in the huge gain in brightness. However, due to its large distance from Earth (1.6 AU), Hubble is unable to spy any pieces that broke off unless they are nearly the size of the nucleus itself. While a theory has been put forward, and Hubble has provided unparalleled detail into the nature of the nucleus, it has not been able to confirm the exact cause of this sudden outburst.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.